Japanese Unexamined Patent Application Publication No. 2006-067234 (Patent Document 1) discloses an antenna that handles a plurality of frequency bands with a single antenna. The configuration of the antenna disclosed in Patent Document 1 will be described with reference to FIG. 1. In the example of FIG. 1, an antenna device 100 includes a dielectric substrate 101, a feeding point 102, a monopole antenna 103, a parallel circuit 104, an antenna element 105, and a parasitic element 106.
In FIG. 1, the hatched portion of the dielectric substrate 101 is a ground of the antenna device 100, and a circuit for performing signal processing of wireless communication is mounted in the hatched portion. The monopole antenna 103 is connected to the feeding point 102. The parallel circuit 104 is connected to the monopole antenna 103. The antenna element 105 is connected to the parallel circuit 104. One side of the parasitic element 106 is connected to the ground in the vicinity of the feeding point 102.
The monopole antenna 103 has a length of approximately ¼ of the wavelength at a frequency f1. The parallel circuit 104 is formed from a parallel resonance circuit constituted by an inductor and a capacitor, which cut off the electrical current of the frequency f1. The antenna element 105, together with the monopole antenna 103 and the parallel circuit 104, has a length approximately ¼ the wavelength at a frequency f2 which is relatively lower than the frequency f1. The parasitic element 106 has a length which is approximately ¼ the wavelength at the frequency f1.
The monopole antenna 103 is connected to the parallel circuit 104, and the parallel circuit 104 is formed of an inductor and a capacitor, which cut off the electrical current of the frequency f1.
In this antenna device 100, the monopole antenna 103 operates by itself at the frequency f1.
As described above, in the antenna device 100, the monopole antenna 103 and the parasitic element 106 operate as an antenna device at the frequency f1. At the frequency f2, the antenna element 105, together with the monopole antenna 103 and the parallel circuit 104, operates as an antenna of a length of approximately ¼.
However, in the antenna device disclosed in Patent Document 1, a parallel resonance circuit is used to cut off the electrical current of a specific frequency f1, and this parallel resonance circuit is inserted into the middle of a radiation electrode. For this reason, the following problems arise:
(1) At the frequency f1, antenna radiation electrodes included in and in a stage subsequent to the parallel resonance circuit are not viewed equivalently, that is, appear to be small. As a result, at a high operating frequency, since the volume of the antenna is decreased equivalently, this is disadvantageous in terms of the performance of the antenna.
(2) Since the antenna operates at a resonance frequency of the lowest Q (with a large loss) of the parallel resonance circuit, an influence due to the loss of the parallel resonance circuit is directly received.
(3) With only the parallel resonance circuit, the electrical current cannot be cut off over the entire range of the high operating frequency. For this reason, interference with generated harmonics occurs, and the performance is deteriorated.
(4) In a case where a three-dimensional radiation electrode is to be formed, rather than the radiation electrode being formed by an electrode pattern on a substrate, a parallel resonance circuit needs to be inserted into the middle of the radiation electrode. In consequence, the radiation electrode needs to be divided in the middle and also, elements need to be directly mounted on the radiation electrode. Therefore, many difficulties are caused to occur in the manufacture of the electrode.